Transforming growth factor-β1 (TGF-β1) directs class switch recombination (CSR) to IgA isotype, which is a predominant antibody in mucosal surfaces. Although IgA is preferentially committed in mucosal lymphoid tissues, it is not definitely established whether hallmarks of IgA CSR such as IgA germ-line transcripts (GLTα), post-switch transcripts (PSTα) and circle transcripts (CTα) are readily expressed in such tissues. Therefore, we compared the expression of these transcripts among mouse Peyer's patches (PP), mesenteric lymph nodes (MLN), and spleen.

Levels of GLTs, PSTs and CTs were measured by RT-PCR in isolated PPs, MLNs and spleen cells.

GLTα and PSTα were well expressed in PP and MLN cells but in spleen cells. Similar patterns were observed in the expression of GLγ2b and PSTγ2b. On the other hand, these transcripts were only inducible in spleen cells upon stimulated with LPS and TGF-β1. In addition, CTα and CTγ2b were detected in PP cells.

PP B cells readily express IgA GLT, PST, and CT. Overall expression patterns of these transcripts were similar in MLN cells. Thus, these results suggest that microenvironment of PP and MLN influences spontaneous IgA CSR, which lacks in systemic lymphoid tissues such as spleen.